Methylenebis(phenyl isocyanate) is commonly used in the production of cellular and non-cellular polyurethanes. Substantially pure methylenebis(phenyl isocyanate) [i.e., methylenebis(phenyl isocyanate) containing less than 5% by weight of polyisocyanate of functionally greater than 2, and including 4,4'-methylenebis(phenyl isocyanate), 2,4'-methylenebis(phenyl isocyanate), and mixtures thereof] is normally solid at room temperature. In general, the substantially pure material must be heated to a liquid state and must be maintained at the heated temperature during processing. Thus, the art has looked to a variety of means of producing modified methylenebis(phenyl isocyanate) which are liquid and storage stable.
One approach suggested in the art has been to heat the substantially pure methylenebis(phenyl isocyanate) in the presence of a trialkyl phosphate to produce a liquid, partially carbodiimidized isocyanate product. See, e.g., U.S. Pat. Nos. 3,384,653 and 3,449,256.
It has also been proposed that heating methylenebis(phenyl isocyanate) alone, in the absence of any catalyst, produced a partially carbodiimized isocyanate which crystallizes upon standing. See, e.g., U.S. Pat. No. 3,152,162. It is believed that this is due to the uncontrolled reaction occurring during the heating step.
Finally, it has been proposed to form liquid, partially carbodiimized isocyanates by using phospholine oxide catalysts in combination with catalyst poisons. In one embodiment, the isocyanate is heated to below 150.degree. C. in the presence of from 1 to 10 ppm of phospholine oxide. The reaction is then terminated by addition of one or more of the halides of hydrogen, phosphorous, or tin, or an oxyhalide of phosphorous or sulphur. See, e.g., U.S. Pat. No. 4,014,935. In yet another proposal, the isocyanate is mixed with from 0.1 to 100 ppm of a phospholine oxide at a temperature of from 0.degree. to 200.degree. C., and the reaction is then terminated by addition of a catalyst poison. See, e.g., U.S. application Ser. No. 715,092, filed on Aug. 17, 1976 now U.S. Pat. No. 4,088,665.